Cavitation issues----------------HELP

[philpot]

I have had a number of both wooden and aluminium floor SIBs but recently bought an Excel SD330 flat air floor which is outside my comfort zone. The engine is a 15hp Parsun with a standard shaft ( 18'' from transom to cavitation plate ).

I have so far only had one other person on board with them sitting forward and myself on the tiller. The boat and engine runs as sweet as can be at half throttle but when we open the throttle three quarters to full, cavitation is the result. If I drop the throttle back to half or there abouts, no problems. The venue is a river with very very little flow so almost still and no wave action.

The cavitation plate is approx 3'' below the transom with the upper plate about half an inch above the transom bottom. I have tried each trim position on the motor without improvement.

The boat pressures are .25 for the tubes, .40 for the keel and .70 bar for the floor. I use a Bravo BTP12 to pump the boat up and have found that a separate pressure gauge shows the floor at .60 and the hand pump which came with the boat shows just below .60 bar so perhaps the Bravo is not quite correct.

So far I have not tested the pressure of the floor, keel or tubes after being on the water for a short time which I will do next time I go out. I do wonder if the floor pressure is critical and if the pressure drops after being on the water for a while.

As I have already said, with this SIB/removable air floor, I have no experience so I am hoping that some words of wisdom will be forthcoming.

Phil

[Landlockedpirate]

The floor and keel pressure are absolutely critical to the boats performance. Even if you have it perfectly inflated before you go in, the moment it hits the water the pressure will drop. Put it in the water for about 15mins then inflate to the correct pressure.

But even with the correct pressure, some sibs still have a habit of cavitating at speed with the biggest and heaviest engine options. Its because the floor isnt completely rigid and has a habit of drawing air in at the front and then dumping out of the back to cause cavitation. Even hard floor sibs can do this as well.

[Peter_C]

Have the passenger move back. It could be they are raising the stern too high.

[dave3235]

Originally Posted by Peter_C

Have the passenger move back. It could be they are raising the stern too high.

Hi all

Sounds like the engine needs to be raised up and up

[Poly]

When you say it cavitates can you clarify the symptoms? It is a commonly misused description for all sorts of things...

[jyasaki]

Originally Posted by dave3235

Sounds like the engine needs to be raised up and up

Can you explain why you think that?

If the OP is getting ventilation, barring any other changes, moving the engine up will worsen the problem, won't it?

jky

[philpot]

Thank you for your replies.

I must confess, I don't understand the idea of raising the motor, if anything, I would have thought lowering the engine by removing some of the transom would be better.

The effect that I describe as cavitation is the engine revs up as though it is not in water. Then winding the throttle right back and turning the throttle back to about half throttle seems to allow the engine to bite again. The process is repeated over and over as we worked our way down river.

Phil

[Poly]

Originally Posted by philpot

The effect that I describe as cavitation is the engine revs up as though it is not in water. Then winding the throttle right back and turning the throttle back to about half throttle seems to allow the engine to bite again. The process is repeated over and over as we worked our way down river.

Did you look at the prop at the time (not that easy to do - and don't hang over the back!) but its usually obvious if there is a big air pocket behind the boat - compared to a nice "normal" wake.

It doesn't make sense that your engine sounds like it is 3" too low on the transom but has ventilation (not cavitation).

Certainly worth making sure that everything is inflated right. But if you are going to do another test run then I'd also make sure the prop isn't "spun" at the same time (see here: http://www.rib.net/forum/f36/mariner...tml#post635201)

[chipko]

You're right it is confusing, but you say AV plate is presently 3" below keel, which would normally be to deep.

[dave3235]

Hi all

You say your engine size is 18" and your transom is 15 I take it.Moveing weight to the back will make the engine deeper in the flow of water.Also is the prop straight if yes check for spun hub.Is it ventilation or cavitation anyway wheres Locozodiac would be interesting to have an explanation.checked my prop because of similar probs only no where near as bad turned out one of the blades is off pitch and my engine length is 17" still working through it though.

PS is your cav plate visible when on the plane.

[philpot]

I have just checked the prop with the engine in gear, no movement at all. I should say it is a brand new engine having done only 6 hours. I take it that by just putting the engine in gear whilst on the engine stand is what you are referring to.

I did look at the prop whilst the high revs kicked in and also while running normally but to be honest I am not sure what was going on down below as the river is not wide so you need to keep a close eye on hitting the side. I could at times just about see the cavitation plate and I do mean just. The boat still moves when the engine revs up but it is something I need to check further.
Any suggestion put forward, I will try to carry out when next on the water, perhaps during this week.

I did take a photo on the phone of the prop position while on the wheels which I will try to post shortly.

Phil

[lightning]

Try raising the engine a couple of CM on the transom with a block of wood. This made a big difference on my boat.

Also my SIB did not want to get on the plane on a river, but performed fine on a lake. Not sure why this would be.

[philpot]

Here is the photo of the transom and engine position.




I am grateful for all your views.

Phil

[dave3235]

Hi phill
Do you run your engine trimmed up at all, or at 90°.Your engine may not like being ran trimmed up.Engine is a bit to deep

PS what size and pitch is your prop.

[philpot]

I was running at 90 degrees Dave but have tried in all the preset positions on my last session on the water.
I have just looked at the prop which is printed 3-91/4'' x 11''. Not sure what the three is unless it means 3 blade but that is pretty obvious.

Do you think 3'' from cavitation plate to transom is too deep, if so, what distance do you think it should be as a test.

Phil

[dave3235]

Originally Posted by philpot

I was running at 90 degrees Dave but have tried in all the preset positions on my last session on the water.
I have just looked at the prop which is printed 3-91/4'' x 11''. Not sure what the three is unless it means 3 blade but that is pretty obvious.

Do you think 3'' from cavitation plate to transom is too deep, if so, what distance do you think it should be as a test.

Phil

Hi

Whilst sorting things out keep at 90°.Raise the engine with block of wood making sure it is secure and you are happy it won't fall off, also correct air presure on tubes and floor.

The only way to check for a spun hub is to take prop off mark across prop bush and through hub body on back of prop then take it for a quick run under load, the line will not match if prop is spun.

[philpot]

Ah, okay Dave, I'm with you now, never had to do this before so a little uncertain how it was to be carried out.

I will test all the things out that have been suggested with luck sometime this week.

Thank you guys

Phil

[dave3235]

Originally Posted by philpot

Ah, okay Dave, I'm with you now, never had to do this before so a little uncertain how it was to be carried out.

I will test all the things out that have been suggested with luck sometime this week.

Thank you guys

Phil

Good Luck

[jyasaki]

Originally Posted by philpot

I have just looked at the prop which is printed 3-91/4'' x 11''. Not sure what the three is unless it means 3 blade but that is pretty obvious.

Yes, 3 blade, 9.25" diameter, 11" pitch.

From your pics, I'd say the motor is too deep (Loco would disagree.) But, as a first step in figuring it out, you need to get the tubes and keel (and floor if applicable) up to the correct pressures, otherwise you're troubleshooting a setup that will be all over the place anyway.

Once there, see how the boat performs as is, then start making small changes and see what improves the situation and what makes it worse.

Pics didn't show: Is your motor bolted on or clamped? If clamped, it's easy to adjust (at least upwards, which I suspect is what you'll need); if bolted, it's a little more work but still do-able.

jky

[nugent]

100% to low in the water. Needs raising for sure